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Battery questions

  1. Dec 4, 2007 #1
    Hey guys,

    I'm new here, and I know just enough to be dangerous, so please bear with me when I ask stupid questions.

    Perhaps like these.

    1) If I understand correctly, a "12" volt automotive battery is made up of 6 cells, each providing 2.2 volts, thus supplying an actual total of 13.2 volts from a "12" volt battery. Is this right?

    2) If I understand correctly, an automotive battery is designed to provide maximum amps for a minimum time (starting the car), at which point it is recharged. If completely drained and then recharged, it's life will be about 200 cycles. A deep cycle battery is designed to be cycled many 100's of times, at (I'd assume) some lesser amount of amps. If this is all correct, what is the difference in design?

    As I said, I know just enough to be dangerous. For example, I always test car batteries by putting my tongue on them...:rolleyes:
     
  2. jcsd
  3. Dec 5, 2007 #2

    Danger

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    You could save yourself the trouble and just try some of Turbo's salsa...

    I think that an auto battery is referred to as '12 volt' simply because that's the system requirement. I don't know about deep-cycle batteries.
     
  4. Dec 5, 2007 #3

    berkeman

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    Welcome to the PF, Wagon Master. Here's an introductory article from wikipedia.org about car batteries, with links to other information sources:

    http://en.wikipedia.org/wiki/Car_battery
     
  5. Dec 5, 2007 #4
    Thanx berkeman,

    The Wiki article pretty much answered my questions. Which leads to a follow up question:

    My father has a solar system that uses 20(ish) deep cycle batteries for power storage. These are fairly expensive to replace when they need it, and take up a fair amount of space on a battery rack.

    Why couldn't he and I get an oversized Rubbermaid container, weld some plastic seperator panels into it, and have our very own really big battery? It would be repairable, with the ability to swap bad plates for good, and be far more space efficient. We could micromanage the electrolyte content, and even build for different voltages (2.1 V per cell times how many cells do you want).
     
  6. Dec 7, 2007 #5
    Any ideas on the build-our-own-battery question?
     
  7. Dec 7, 2007 #6

    Integral

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    Sounds like an accident waiting to happen, not in my house.

    How could you make it more space efficient then a modern auto battery? There is a lot of money being spend in battery research now. You will be in competition with teams of engineers with corporate level funding, all trying to make a better battery. The chances of a layman individual contributor making a break though in battery technology is remote. Meanwhile a large vat of acid setting around the house has little attraction for me.
     
  8. Dec 7, 2007 #7
    I'm not looking to re-invent the battery, just the package. 20 current issue deep cycle batteries, with all the attendant shelving and cables, take up a lot of room are far less space efficient than what I propose. I'm not really keen on the acid in the house either, but out in a shed (where the current batteries are stored) is another story.

    I'm just wondering if there are any obvious (to you guys who know such things) flaws in the idea of creating a physically larger battery than is currently for sale?

    Thanks
     
  9. Dec 8, 2007 #8

    chroot

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    I'm an electrical engineer, and I really, really doubt that your rack of deep-cycle batteries could be made more space efficient by a guy with a Rubbermaid vat, acid, and some metal plates. I agree with the others -- please do not try this. At best, it won't work; at worst, someone will get hurt.

    Also, note that a shelf full of batteries wired in series creates a potential difference of some hundreds of volts. Such voltages can be deadly, and can vaporize dropped wrenches. The hazards are numerous.

    That said, larger batteries do exist -- utility companies and commercial UPS systems use them, for example. They're probably pretty damned expensive, though.

    - Warren
     
  10. Dec 8, 2007 #9

    Danger

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    It seems a bit strange to me that the present arrangement is too 'bulky', but the bit about all of the 'attendant cables' might be the main problem. I'd be using flat-strap bus bars for the intra-battery connections, with the batteries physically stacked together like bricks. If that were to cause a thermal problem, they could be separated an inch or so with cooling fans blowing through the gaps. It should be a lot more compact than a shelving system.
    Am I missing something?
     
  11. Dec 8, 2007 #10

    Sounds good until the one on the bottom needs to be replaced :smile:. That's where a rack with about 4" of space above each battery comes in handy.
     
  12. Dec 8, 2007 #11

    Danger

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    Gotcha.
    I did actually think of that when making the suggestion, but dismissed it because I figured that maintenance-free (sealed) batteries wouldn't need to be accessed.
    Still, the batteries could be stacked end-to-end, with bus bars, on slide-out racks similar to cabinet drawers and still be very compact. You could, for instance, have 5 batteries in one rack (connected with bars) with an identical rack beside it and 2 more on top. The rearmost batteries in each rack can be connected with plug/socket units. For maintenance, an entire rack could be pulled out on its rails (unplugging from the neighbours), then pushed back in when ready to go.
     
  13. Dec 8, 2007 #12
    Warren,

    A motorcycle battery is no different than a car battery, just smaller. A group 24 battery is smaller than a group 78 battery. A marine battery is larger than all of them, still no different other than size. According to Wiki, a modern battery is really nothing more than a plastic box with some dividers in it, and acid poured over some metal plates. Commercial outfits (as you state) have batteries that are really no different than a car battery, just many times the size.

    I'm not trying to be arguementative; it's just that 99% of the population will tell me it can't be done, but when pressed for a reason, will only say "beacause nobody has done it". I came to the Physics Forums to find out why it can't be done.
     
  14. Dec 8, 2007 #13

    chroot

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    You are absolutely incorrect.

    Marine batteries are absolutely different from motorcycle batteries, in more ways than simply the physical size. Marine batteries have much larger capacity, and can deliver much greater currents. (They have thicker plates and thus smaller series resistance.)

    You can try the acid-vat technique if you want. I'm not saying it can't be done -- I'm just saying it probably won't work as well as commercially-made batteries and will subject your lives and property to significantly greater risk. The risk/reward ratio on this is just way too high to be a sensible project. Do it if you want, but don't say I didn't warn you.

    - Warren
     
    Last edited: Dec 8, 2007
  15. Dec 8, 2007 #14

    I don't know about absolutely incorrect: The motorcycle to car battery comparison seemed good, and while deep cycle batteries have a different output (lower over a longer period of time), their construction is basically the same.

    The advantages of the acid-vat over a commercial battery are cost (I'd assume: I haven't actually priced anything yet, because until today I wasn't sure it could be done.) and user servicability. My father is 80 miles of mud from the nearest place to get a commercial battery.

    As for the warning, thank you. And not to worry; I would never blame someone else for not warning me about a choice I made as an adult.
     
  16. Dec 8, 2007 #15

    Danger

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    First, check out Rubbermaid's acid resistance. You could end up with a big puddle of molten plastic.
    At the very least, make sure that you provide a proper ventilation system. Your homemade battery is pretty much sure to exhale deadly fumes.
     
  17. Dec 8, 2007 #16

    Rubbermaid wouldn't be the actual container; I just used it for an example. As for the deadly fumes, agreed. Though somehow many of today's batteries are completely sealed...:confused:
     
  18. Dec 8, 2007 #17

    OmCheeto

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    how'd i do on my first try?

    It can be done. Your idea will work. Though, as several have pointed out, it will be dangerous. And I would add that it may not even be cost effective. Now it's been 30 years since I've studied industrial sized lead acid batteries so some of my opinion may be outdated.

    Lead acid batteries, as I recall, use something called sponge lead. Sponge lead increases the surface area of the plates giving you a higher capacity. Replacing sponge lead with simple lead plates would require that you use more plates to get the same capacity. Now if the plates are too thin then you run into the problem of plate disintegration. As you know, when you discharge a lead acid battery, the lead leaves the plate and enters the solution. As I recall, this was the primary source of battery failure, ie. entire chunks of the sponge lead crumbling off of the plates due to uneven lead removal and redeposition.

    I believe this is why you don't want to discharge lead acid batteries past a certain point. My recommendation would be to double or triple the number of batteries in your system to prolong their life. If you still want big batteries, I would research the type they use in submarines. Each cell weighs about a ton, and of course you would require 6, so it might be a bit spendy.

    I could go on all day about this, so I'll end with a little autobio stuff since this is my first post and this might explain why I started on this thread:

    I have 4 solar panels. Mainly I use them in my boats as an auxiliary power source. I live in a forest setting and have been too chicken to climb up the 60 or so feet to get them into some decent sunlight. I had 2 panels set up on the south side of my house last winter and could not get enough solar energy to keep my battery charged to run a 1 watt string of x-mas lights.

    I guess I have just the opposite problem that you do. Even with all 4 panels in place, I wouldn't even keep a 12 volt garden battery charged.

    Has anyone here at the forum done the math on the difference between compressed air energy vs. battery storage capacity as a function of cost/volume/efficiency/etc.? I know they run buses in Brazil using compressed air. Just thought it might be a viable alternative.
     
  19. Dec 8, 2007 #18

    Danger

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    Welcome to PF, OMCheeto.
    That was a beautiful first post. Very informative, without being condescending. That indicates to me that you will be a very valuable asset to these forums.
    In fact, I learned a lot from that. I never realized that the lead plates were sponged. It makes a lot of sense, but I never heard about it before.
     
  20. Dec 8, 2007 #19

    Integral

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    Just as a reminder of your own words at the start of this thread. I hope that you will begin to understand that there is a significant amount of engineering invested in a modern battery.

    If you must build a battery, please start small.
     
  21. Dec 8, 2007 #20

    OmCheeto

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    Thank you. But before you get too comfortable believing what I say, let me just say that there is an error in my first post. I've been researching compressed air for the last hour and have decided that my brain has somehow short-circuited. I can find no reference to air powered vehicles in Brasil anywhere on the net. Perhaps it was just a dream I had one night, or perhaps I read it in a kook article.
     
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